The present invention relates to an improved developing apparatus for an electrostatic reproduction machine.
In conventional xerography, a xerographic plate (photoreceptor) comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., paper) and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photoreceptor surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photoreceptor to electrostatically attract the toner image from the photoreceptor to the paper.
A modern high speed duplicating machine uses a developing apparatus having magnetic brushes to transport developer (carrier plus toner) from a developer sump to the photoreceptor to develop the latent image. Toner is periodically dispensed into the developer sump by a toner dispensing device, the latter being actuated by a sensor and control means in accordance with the sensed density of toner alternately deposited on either of two attracting elements. The amount of useful toner particles in the developer can be determined by the amount that will be alternately deposited upon the elements, each being charged with a voltage to set up the proper field between the elements. This toner determination is utilized to control the amount of active toner within the apparatus. The sensor, having two elements each capable of carrying a charge placed thereon, is positioned within the apparatus to receive some of the developing material falling between the elements. Potentials are alternately placed upon the elements cyclically, thereby reversing the electric field between the elements cyclically. This causes toner to be attracted to and cleaned from the elements cyclically. While one element is provided with an attracting field, the other element serves as a development electrode for the solid area development of the attracting element since the electric field between the elements will be uniform. The amount of toner attracted to each element when it is charged to attract toner for any particular period of time is related to or a function of the developability in the developer apparatus. The elements are connected in an electrical circuit which produces a steady state signal set for optimum condition and which, when deviated from a preset level, generates a control signal introducing toner particles into the developer sump. Such a sensor and control means is thoroughly described in U.S. Pat. No. 3,727,065 assigned to Xerox Corporation, the disclosure of this patent being incorporated by reference herein.
Included in the above described developing apparatus is a screen through which the developer passes before reaching the sensor, the screen separating contaminates from the developer before the latter reaches the sensor. One of the problems with this arrangement is that the screen becomes clogged with developer after only a few thousand copies are made. When the screen becomes clogged, very little or no developer reaches the sensor, and this causes the sensor to actuate the toner dispenser an excessive amount. Consequently, an excessive amount of toner is dispensed into the developer sump, and this eventually causes the machine to go into a failure mode in which copy quality deteriorates, the machine cleaning system fails, photoreceptor filming occurs, etc. This problem is magnified when a duplicating machine is operating in areas of low humidity.
The applicants discovered that the reason the developer was accumulating on and clogging the screen was because the developer was becoming excessively charged. Upon discovering this, attempts were made by others (without success) to discover the reason for the excessive charging. The applicants eventually discovered that the reason the developer was becoming excessively charged was because an electrically isolated guide plate in the developing apparatus was becoming excessively charged and eventually caused the developer to become excessively charged through direct contact. After this became known, others suggested that either electrically biasing the guide plate or grounding the plate would solve the problem. Neither suggestion proved successful.
After further investigation, the applicants then discovered that the spacing between the guide plate and one of the magnetic brushes was critical because of this spacing within extremely close tolerances, the charge on the guide plate would discharge to the magnetic brush before the magnitude of the charge became sufficient to cause the developer to clog the screen. The applicants eventually discovered, however, that this spacing could not be controlled to the degree necessary because variants in the manufacturing process actually caused direct contact between the magnetic brush and the guide plate most of the time when the spacing was adjusted to the extremely close tolerances necessary. Consequently, the applicants concluded that what was necessary was to control the magnitude of the charge build-up on the guide plate by other means so that the spacing between the plate and the magnetic brush only had to be nominally adjusted.